Transition from Cage Clathrate to Filled Ice: The Structure of Methane Hydrate III

The structure of a new methane hydrate has been solved at 3 GPa from neutron and x-ray powder diffraction data. It is a dihydrate in which a 3D H-bonded network of water molecules forms channels surrounding the methane molecules. The network is closely related to that of $\mathrm{ice}-\mathrm{I}h$ and the methane-water system appears to be the first in which a cage clathrate hydrate is transformed into an ice-related hydrate (a “filled ice”).